Cutting metallates of refractory metals

ABSTRACT

Lithium niobate and tantalate crystal rods or boules are sawed to wafers with diamond edged saws in the presence of inert fluorinated lubricating liquids.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for the sawcutting and other cutting of metallates of refractory metals including:niobates, tantalates, titanates, and more particularly crystals oflithium niobate and lithium tantalate and to fluids used for frictionreduction, swarf removal and/or cooling in such processes (“lubricatingfluids”).

Several Ta, Nb, Ti metallates have found significant usage as materialsfor manufacture of solid state power, signal translation, transducingand sensing devices, including usage as filters, resonators, delaylines, piezoelectric transducers. They are also useful in connectionwith the making of perskovite structure ceramics, including theso-called PMN types (e.g., lead-magnesium-niobate, with well-knownsubstitutions for each such nominal component) useful for ferroelectricand dielectric properties.

The class of metallates involved here is glasslike materials availableand usable in polycrystalline and single crystalline and in amorphousforms, usually grown or refined as single crystals by Czochralskiprocess pulling and growth from a melt or Pfann zone refining process.The crystal boules or rods vary from a few cm to well over a foot indiameter. It is desirable for most usage purposes to cut them into waferthin slices, with dimensional control, achieve very thin sections, avoidexcessive kerf and swarf, suppress impurity pick-up, suppress surfacewear and roughness, recycle swarf and avoid breakage or crystalline ormechanical structural flaws of the wafer to a high degree. Maintainingcutting tool life and precision, and control of lubricant waste anddisposal are also important criteria.

The principal cutting method presently used for such materials is sawingusing rotary metal blades with an edge coating composite of diamondparticles in a polymeric or metal blade matrix (diamond saw), usingwater-soluble or emulsifible oils (in aqueous solutions or emulsions) aslubricants for cooling, removal of swarf and friction reduction.Sometimes water, per se or with minimal functional additives, is used asthe lubricant. Generally a wafer thickness as low as 0.5 mm isachievable. Cutting is followed by deionized water rinsing of thewafers, and air drying, to further remove swarf and residual lubricant.Filtering is used to recover the swarf particles (often sub-micronsized) and the lubricant is recycled or disposed in an environmentallysound way, at a significant cost.

The objects of the present invention are to provide improved method,apparatus and lubricant systems and materials to enhance blade life andprecision, enable greater speed of cutting, reduce swarf and otherwaste, reduce wear and roughness of the wafer surfaces, eliminatesubsequent water rinsing, increase yields and/or reduce the thicknesslevel regularly achievable with the metallate materials, particularlylithium niobates and tantalates, as well as reducing costs.

SUMMARY OF THE INVENTION

The objects of the invention are realized through method and apparatusemploying a class of lubricants and systems for their utilization in thepresent context that can be recycled without complex or expensivereconstitution, cools more effectively than state of the art lubricantsand enables closer control of the cutting. The invention can also beextended to other operations concerning the metallates including surfacegrinding and polishing, as well as the cutting of strips and dice fromthe wafer slices. It has been found that diamond blade wear can bereduced remarkably—to a degree that useful blade life is extendedmanyfold, that rpm, tool infeed rate and thereby speed of cutting can beincreased.

The preferred processes and machines employ a lubricant comprising oneor more of: (a) perfluorocarbon compounds (PFCs), including aliphaticperfluorocarbon compounds (α-PFCs) having the general formulaC_(n)F_(2n+2), (b) perfluoromorpholines (PFMs) having the generalformula C_(n)F_(2n+1)ON, (c) certain perfluoroamines (PFAs), (d) highlyfluorinated amines (HFAs), (e) perfluoroethers and perfluropolyethers(PFEs and PFPEs), (f) hydrofluoro polyethers (HFPEs) and (g) highlyfluorinated ethers (HFEs), and their respective polymerization products.Such compounds exhibit a very high degree of thermal and chemicalstability due to the strength of the carbon-fluorine bond. PFCs are alsocharacterized by extremely low surface tension, low viscosity, and highfluid density. They are clear, odorless, colorless fluids with boilingpoints from approximately 30° C. to approximately 300° C. (although thepreferred boiling range for refractory metal matellate cutting is 30-200C., and more preferably, as stated below, 50-150).

Fluorinated, inert liquids usable in accordance with the presentinvention can be one or a mixture of α-PFC, PFM, PFA, HFA, PFE, PFPE,HFPE and HFE compounds having 5 to 18 carbon atoms or more and having aH:F ratio under 1:1. When any of these choices other than ethers areused, they preferably have a hydrogen content of less than 5% by weight,most preferably less than 1% by weight. When ethers are used, a hydrogencontent of less than 2% is the most preferred.

These materials are preferably used in liquid phase and used alone(neat), but in some usages may usefully be mixed or emulsified withother functional or carrier liquids and/or mixed with particulate solidsas pastes or waxes. They can also have useful solids suspended thereinwhen used in liquid form.

Suitable fluorinated, inert liquids useful in this invention may includemore particularly, for example, perfluoroalkanes orperfluorocycloalkanes, such as perfluoropentane, perfluorohexane,perfluoroheptane, perfluorooctane, perfluoro-1, 2-bis (trifluoro-methyl)hexafluorocyclobutane, perfluorotetradecahydro-phenathrene, andperfluorodecalin; perfluoroamines, such as perfluorotripropylamine,perfluorotributylamine, perfluorotriamylamine perflurotriethylamine,perfluoromorpholines, such as perfluoro-N-methylmorpholine,perfluoro-N-ethylmorpholine, and perfluoro-N-isopropylmorpholine,perfluorophenanthrene and perfluoroethers and perfluoropolyethers, suchas perfluorobutyltetrahydrofuran, perfluorodibutylether,perfluorobutoxyethoxyformal, perfluorohexyl formal, andperfluorooctyl-formal, perfluro polyethers and the polymerizationproducts of these classes.

The prefix “perfluoro” as used herein means that all, or essentiallyall, of the hydrogen atoms are replaced by fluorine atoms.Perfluorocarbon fluids originally were developed for use asheat-transfer fluids. They are currently used in heat-transfer, vaporphase soldering, and electronic testing applications and as solvents andcleaning agents and have also been described in connection with usage incertain shop operations such as: wire drawing, hot and cold bulkforrning, cutting and abrasion processes see, e.g., U.S. Pat. Nos.5,676,005 (Oct. 14, 1997) and 5,743,120 (Apr. 28, 1998) and publishedPCT application WO097/35673 (published Oct. 2, 1997) of H. C. Starck,Inc. (Newton, Mass.), as well as the PCT published PCT applications ofD. S. Milbrath et al. of 3M Co. (Minnesota Mining & Manufacturing Co.,Inc., St. Paul, Minn.), WO9812286 and 9812287 published Mar. 26, 1998,based on U.S. applications Ser. No. 08/715,207 and 08/715,206 of Sep.17, 1996.

The term “highly fluorinated” as used herein means having a H:F ratiounder 1:1. Commercially available fluorinated, inert liquids useful inthis invention include the PF-5062, HFE7100, HFE7200, FC-40 liquids (allavailable from 3M Company under the trade-name designations of“Fluorinert,” performance fluid or hydrofluorether as described in 3M'spublished PCT patent applications. Other perfluorocarbon liquids suchas, HT-200, HT-230 and HT-270 (available from Montefluos Inc., Italy,under the tradename designation of “Galden”); Hostinert™(Hoechst-Celanese); and Krytox brand K—101, 103, 105, 107 (DuPont) arealso usable for purposes of the present invention.

Generally, fluorinated liquids of the invention should be selected for a50-180° C. boiling point for most metallate grinding and polishingapplications: Liquids with higher or lower boiling points are likely tosuffer from degraded performance such as lower cutting ability above3,000 rpm and reduced infeed rates or high loss rates, respectively,over this preferred boiling point range.

The lubricant can be filtered between usages for a defined series ofcuts to trap fine particulate swarf from cutting and make the liquidavailable for reuse, as well as recovery and reuse or proper dispositionof the particles. Through the present invention the intervals betweenfilter changes can be extended substantially since there are fewer andsmaller particles produced in the practice of the present inventioncompared to prior practice using aqueous lubricants.

If very low boiling PFC lubricants are used, then the vapors should becondensed and returned to liquid form lubricating usage in the same or asubsequent cycle of cutting. Additionally or alternatively, the workenvironment can be cooled to limit vaporization. The cutting can becarried out with the diamond wheel initially coated at least at itscutting surface with lubricant and with a nozzle feeding a stream oflubricant to the cut region or blade or with the cutting tool andworkpiece all immersed in a bath of the lubricant. After cutting thewafer slices can be, but need not be, washed with non-aqueous cleaningfluids to remove residual fluorinated lubricant. In some instances aclean, inert gas should be blown over the wafer surface to carry awaysub-micron particles and aid evaporation of residual lubricant.

Surface roughness of the refractory metal metallates will besubstantially less in processing through the present invention comparedto state of the art processing. The roughness is expressed in an averageof projection heights (in nanometers, nm) determined by scanning probemicroscopy (SPM). This technique generates a digital topographic imageof the surface of the sample. The vertical movement of the probe isaccurately measured by means of a laser and recorded. These data areanalyzed to develop a three dimensional representation of the sample'ssurface and a quantitative measure of the surface roughness at aneffective magnification of 1,000X. Smoother samples have a lower surfaceroughness measurement.

The metallates treatable advantageously through the present inventioninclude niobates, tantalates, titanates, vanadates, hafnates,zirconates, molybdates and tungstates of the metals lithium, barium andother suitable metals.

Other objects, features and advantages of the present invention will beapparent from the following detailed description of preferredembodiments thereof, taken in conjunction with the accompanying figuresof the drawing, in which:

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1-3 is an SPM image of a lithium tantalate sample sawed using,respectively, water (prior art) and then perfluorotributylamine andperfluro-N-methylmorpholine per the present invention all at 3,000 rpmand fast feed;

FIGS. 4-6 are a similar SPM runs at slower infeeds, and an addition(FIG. 7) using 1-ethoxy-nonafluoro-butane;

FIGS. 8-13 are a similar SPM series for lithium niobate at high speedfeed and 3,000 rpm using as lubricant, (8) water; (9)perfluorotributylamine; (10) perfluro-N-methylmorpholine; (1)1-ethoxy-nonafluoro-butane; 10 methoxy-nonafluoro-butane (12);perfluorotriamylamine (13), and

FIGS. 14-18 are similar SPMs at slower in-feed for: (14) water, (15)perfluorotributylamine, (14) perfluorotriamylamine, (15)perfluorotributylamine, (16) perfluoro-N-methylmorpholine, (17)1-exthoxy-nonafluoro-butane, (18) 1-mexthoxy-nonafluoro-butane.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is now described further with reference to workingexamples of its usage, the examples being provided as illustrative ofand not in limitation of the invention.

EXAMPLE 1 Lithium Tantalates

An 0.875 inch diameter rod of lithium tantalate crystal was cut with anAccutom-2 laboratory scale diamond cutoff saw, using as lubricant indifferent runs, water and several forms of perfluorinated inert liquidsto form 0.5 mm thick wafers. The lubricants were sprayed on the blade tofully wet it. The time of cutting through and rpm were varied and inmost instances surface roughness were determined on a cut side of eachresultant wafer (same side from sample to sample). The results are shownin Table I for Example I below, where cutting time (time) is shown inminutes: seconds, cut length (length) in inches and surface roughness(roughness) in nanometers.

TABLE I Lithium Tantalate Cutting Tests Cut Surface Roughness Blade FeedTime Length RMS/nanometers Lubricant RPM (min:sec (inches) FIG. Water3000 0:18 0.875 1836 Perfluorotributylamine 3000 0:22 0.875 2460Perfluoro-N- 3000 0:22 0.875 3759 morpholine 1-etholy-nonafluoro- Nodata butane Water 3000 3:26 0.875 4566 Perfluorotributylamine 3000 3:160.875 5415 Perfluoro-N- 3000 4:33 0.875 6712 morpholine1-etholy-nonafluoro- 3000 4:35 0.875 7606 butane

Example 2-Lithium Niobates

An 0.875 inch diameter rod of lithium niobate crystal was cut with anAccutom-2 laboratory scale diamond cutoff saw, using as lubricant indifferent runs, water and several forms of perfluorinated inert liquidsto form 0.5mm thick wafers. The lubricants were sprayed on the blade tofully wet it. The time of cutting through and rpm were varied and inmost instances surface roughness were determined on a cut side of eachresultant wafer (same side from sample to sample). The results are shownin Table 11 for Example 2 below, where cutting time (time) is shown inminutes: seconds, cut length (length) in inches and surface roughness(roughness) in nanometers.

TABLE II Lithium Niobate Cutting Tests Cut Surface Roughness Blade FeedTime Length RMS/nanometers Lubricant RPM (min:sec (inches) FIG. Water3000 0:16 0.825 81047 Perfluorotributylamine 3000 0:16 1.000 9651Perfluoro-N- 3000 0:21 0.875 10615 morpholine 1-etholy-nonafluoro- 30000:24 1.000 11629 butane 1-methoxy-nanafluoro- 3000 1:01 0.875 12425butane Perfluorotriamylamine 3000 Broken 13703 Water 3000 3:36 0.87514589 Perfluorotributylamine 3000 3:45 1.000 15717 Perfluoro-N- 30001:25 0.875 16667 morpholine 1-etholy-nonafluoro- 3000 4:41 1.000 17468butane 1-methoxy-nonafluoro- 3000 4:31 0.875 18213 butanePerfluorotriamylamine 3000 Broken N/A

It is thus seen that a substantial advance is made in the art ofmetallate materials. They can be cut to thin wafer forms with goodspeeds and reduced tool wear, consistent with good surfacecharacteristics. Despite breakage during the tests with the aminelubricant, it should not be discarded as a selection and is usablewithin the scope of the invention, as are the other members of the classof inert fluorinated lubricants specified herein.

It will now be apparent to those skilled in the art that otherembodiments, improvements, details, and uses can be made consistent withthe letter and spirit of the foregoing disclosure and within the scopeof this patent, which is limited only by the following claims, construedin accordance with the patent law, including the doctrine ofequivalents.

What is claimed is:
 1. Method of working refractory metal metallatescomprising applying a cutting tool to a workpiece form of the metallatein the presence of one or more of the fluorinated lubricants selectedfrom the group consisting of PFCs, PFAs, HFAs, PFMs, and Polymerizationproducts of any of the foregoing.
 2. Method as defined in claim 1wherein the working method is sawing with a diamond edged wheel. 3.Method as defined in claim 1 wherein the working method is cutting. 4.Method as defined in claim 1 wherein the working method is grinding orpolishing.
 5. Method as defined in any of claims 2-4 and 1 wherein themetallate is in a single crystal elongated form.
 6. Method as defined inany of claims 2-4 and 1 wherein the metallate is selected from the groupconsisting of niobates, tantalates, titanates, molybdates,tungstates,zirconates, hafnates, vanadates.
 7. Method as defined in claim 6 whereinthe metallate is lithium niobate.
 8. Method in accordance with any ofclaims 1-4 wherein the lubricant is selected from the class consistingof perfluoroalkanes, perfluorocycloalkanes, perfluoroamines, andperfluoromorpholines, and the polymerization products of these materialsand substituted products of any of them.